Also, it is more difficult to cool under negative pressure. Pressurizing the case is the optimal solution, i.e. two in one exhaust. Even large cooling expeditions (i.e. radio transmitter sites) positively presurize the building.

pressure should have nothing to do with cooling abilities. cooling will be dependent on the air flowrate, surface and flow area of the components. air temperature, humidity, component temperature, but not pressure. if you want to cool your case effectively, make sure you have the maximim VOLUME going through the case. if i were you, i would use the three fans to PULL air through the case. blowing air into the case would work, but i think that drawing air through it, you will get better results.

more importantly, is the flow path. make sure you have the cleanest flow path for the air to travel. what i mean is make sure you have the components exposed in the airflow, and try to keep wire bundles and other junk out of the way. putting the fans as close to the components as possible will ensure that the maximum air flow goes across the hot stuff.

also, if u are planning on using multiple fans, try to make sure you use fans that have the same abilities. if they are all pulling air through the case, or pushing air through the case, this isnt a big deal. but if you have one pushing air into the case and two pulling air into the case, then you will have a problem b/c the overall system will be choked by the single inlet.

finally, make sure you have enough inlet (or outlet) flow area to allow adequate flow of air into (or out of) the case.

an engineers perspective....
~mike

Single Member of the "1000 Post and No MP3 Car" Club
PROJECT ON INDEFINATE HOLD... BOUGHT A HOUSE
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The push versus pull idea, is pretty much a non issue. However, sucking air out, the air inside, cooling the hot stuff is not turbulent. If you push air in, then however, it is turbulent. Turbulent air is better for absorbing heat than static air.

As for the issue about using 3 fans to push air in, or exhuast it out, other than the turbulency issue, it makes no damn difference.

Technically positive pressure makes denser air, and better cooling... pressure increase from 3 80mm fans is very slim so that aspect is trivial.

Dirt, well I like to control where air enters and donr care much where it leaves, as long as I know it circulates where needed. I filter most of my rig fans with klenex (it catches dust pretty good.)

I'd suggest 3 intakes and a blow hole for a vent. Keep in mind, heat rises, put the blow hole as high as you can. This is kind of hard without knowing what it's all mounted in.

If there are any pockets that air will not circulate through, but have components in them, put a small blowhole there too.

FYI... I did an extensive physics project on this in high school. It was really fun and I still maintain it's was the best in the class. We (small group) played with all sorts of configurations. for temperature, neutral pressure was the best. We used 2 80mm in and 2 80mm out, letting air flow right over the motherboard. But in our thesis, we also considered things like dust, noise, lighting (LED fans were just comming out), etc. If you guys want to have the airflow- fight, I'm game.

As was stated above the whole push/pull thing is a wash, it just don't matter. Either way the same amount of air will flow at the same preasure carrying the same amount of dust.

What does matter is that you have air moving past all the stuff that gives off heat. It's just like water flowing down hill and will take the easiest path. Make sure there is a path by all the heat generators. There are a number of ways to do this using push, pull or combo arangements.

In the last case I built I put two fans blowing out and then i drilled grills in the case at all the locations I wanted the air to come in.

Increasing the case pressure in theory will heat up the air inside, but in this situations its neligible.

Heres my opinion

Pushing air in is better for dust control as it give you the chance to filter the air before it goes in to the PC. Most air will get sucked into the pushing fan rather than through the cracks/holes in the case.

A pull only will suck air all over the cracks/holes such as FDD and CDROM and make the whole PC dusty.

A single fan design, a push or pull will cool the whole case more or less evenly...because it push or pull air all over the place.

A push and pull design will give you the control where most cooling is needed...because you can place the fan where you want and the path between the pushing fan and the cooling fan give the most air flow.

Ill say have more pushing fan than pulling fan and placed them at the right places for effective cooling...no point in flowing air next to the FDD.

Non is scientific but thats how I see it work...so maybe someone can correct me on this.

Sorry to be a jerk but I actually am an engineer and am very proud of it, we are the smartest people on earth you know, if not the best dressed

In any event any engineer would know that unless you are restricting the airflow there will be no pressure change. And even if you were restricting the flow that change would be very very slight and the density effect would be dramatically counteracted by the reduction in flow caused by the restriction.

here's a quick mental exercise letís say your case is 12Ē x 10Ē which would have a 120 square inch lid. Now lets say you had a 3 psi pressure change, pretty slight from the standpoint of the physical properties of the air right?. Well that would mean there was a 360 lb force trying to pop off the top of your case. I wonder if those little 4-40 screws would hold it shut? Also when you add in the bottom 120 square inches and the area from the sides,180 square inches in the case of a 4í tall box, you have a total of 1260 lb of force. Hmmm I donít think that little fan is that strong. Do you?

Ok enough sarcastic jerk. Sorry. If you open up a digikey catalog to the fan section the fans will list a max air pressure along with a max flow. They give these two numbers so you have an idea about how the pump(fan) behaves in different conditions. The max flow is under zero restriction and the max pressure is when it is blowing into a closed box. If you look the numbers are given in mm of H2O. Thatís a pretty small unit. So for example if your average fan can develop 3mm H2O that equates to .004 psi, and thatís at zero flow. Not such a big pressure change is it?